Method and system of high-frequency transmission



July 20 1926.

E. O. SCRIVEN METHOD AND SYSTEM OF HIGH FREQUENCY TRANSMISSION 2Sheets-Sheet'l Filed Sept. 29 1919 //7 1/6 n/ar. [mm/0 0 Sam en.

E. O. SCRIVEN METHOD AND SYSTEM OF HIGH FREQUENCY TRANSMISSION FiledSept. 29 1919 2 Sheets-Sheet 2 IHI HIE

/m/e/7/0r: Edward O. Jeni/en @QXT/ A f/fy UITE STAT I:

A'FNT F F E C E EDWARD o. soRIvEn, OF NEW YORK, N. 2., ASSIGNOR 'roWESTERN ELECTRIC oom- PANY, INCORPORATED, on NEW YORK, N. Y., ACORPORATION OF NEW YORK.

METHOD AND SYSTEE OF HIGH-FREQUENCY TRANSMISSION.

Application filed September 29, 1919. Serial No. 327,039.

This invention relates to methods and systems of. high frequencytransmission and more particularly to multiplex carrier wave signalingin which high frequency reinforcing oscillations of the carrierfrequency are combined at a receiving station with the received carrieroscillations.

An object of the invention is to make possible two-way transmission withthe same .carrier frequency and at the same time to secure an exactcorrespondence in frequency between the carrier frequency of theoscillations received at each station and the reinforcing oscillationsat that station.

A feature of the invention is the derivation of all of the carrier andreinforcing oscillations used in a multiplex system at both terminals ofthe high frequency line or both radio stations, as the case may be, fromcontrol oscillations produced by a single oscillator.

A further feature of the invention is the arrangementwhereby thereinforcing oscillations are applied to the output circuit of auni-directionally conducting detector or demodulator in order to preventtransmission of these reinforcing oscillations to the high frequencyline.

According to the present invention, the different frequency carrieroscillations or carrier waves are produced by first generatingoscillations called base frequency or control" oscillations as. forexample, 5000 cycles. These base frequency oscillations are thendistort-ed to produce harmonic frequency oscillations as, for example.of 10,000, 15.000 and 20.000 cycles, and the harmonic frequencyoscillations are then separated so that oscillations of one particularharmonic frequency may serve as one carrier wave and oscillations ofother harmonic frequencies as other individual carrier waves. It isdesirable to be able to carry ontwo-way communication using waves of thesame carrier frequency for transmission in each direction. This may beaccomplished by transmitting part of the base frequency oscillationsfrom the station where they are produced to the receiving station withwhich communication is to be maintained. At the receiving station. there* ceived base frequency oscillations are distorted iii-the samefmanneras at the transmitting station to produce carrier waves for the returntransmission.

According to the method of carrier wave reception termed homodyne, theamplitude of the signal obtained from the received carrier wave whendetected or demodulated is intensified by combining the modified ormodulated carrier wave oscillations as received with locally generatedreinforcing oscillations of the carrier frequency. This method ofreception is disclosed and claimed in United States Patent 1.330.471.patented February 10, 1920, to B. \V. Kendall. If these locallygenerated or reinforcing oscillations differ in frequency from thecarrier frequency of the received oscillations, beats of the differencefrequency are produced and may be manifested as a sound or noise whichinterferes with a distinct reception of the signal. An important featureof this invention lies in the use of a portion of the carrier waveoscillations produced at a station to reinforce the modifiedoscillations of the same carrier frequency received at that station. Itwill, therefore, be clear that all of the carrier oscillationstransmitted in both directions in the various two-way transmissions of amultiplex system as well as the reinforcing 0scillations supplied to thereceiving circuits are derived primarily from a single source, thebase-frequency oscillator. The power output of the base-frequencyoscillator may, 7

however. be "cry small. since amplifiers operating substantially withoutdistortion are available to increase the magnitude of a fluctuatingcurrent to any desired degree.

It is frequently highly desirable that no oscillations of the carrierfrequency be transmitted through the ether or over the conductingmultiplex circuit, as the case may be, except during transmission ofsignals or control impulses. It is further desirable that during thetransmission of sig nals or control energy, the envelope of the modifiedcarrier oscillations, as transmitted, be directly proportional to thesignal or control impulses or, in other words, that there be nounmodulated component of the transmitted carrier wave. The first ofthese objects, namely, the prevention of transmission of carrierfrequency oscillations, except during intentional transmission, isaccomplished by two different expedients. For the outgoing highfrequency channel, a modulator of the general balanced type disclosed inUnited States Patent 1,343,306, patented June 15, 1920, to J. R. Carson,is used. This modulator is placed between the source of carrieroscillations and the higdh frequency line of transmitting antenna an thehigh fre uency line except as modified or modulate waves varyingsubstantially in direct proportion to the low frequency waves orimpulses impressed on the modulator. For thenincoming channel, aunidirectionally conducting detector or demodulator, referably of thethermionic type, is use and the local source of reinforcing oscillationsis connected to its output circuit and is thus prevented by theumdirectionally conducting characteristic of the detector from supplyinghigh frequency oscillations back through it to the high frequency line.It follows, therefore, that waves of carrier frequency are excluded fromthe high frequency line except during signal or control transmission andthat during such transmission, the envelope ampli-' tude of the highfrequency carrier wave is directly proportional to the signal or controlwave.

This invention is a plicable to telephony, telegraphy or any ot er formof electrical signaling or transmission. Although useful for radiosignaling, it is of particular importance in wire line signaling, notonly in that it enables a number of telephone or other communications tobe carried on simultaneously, but also because this multiplex carriersystem may be applied to present day signaling lines, thus increasingthe number of messages which they may transmit without interferingwith'the ordinary lower frequency signal currents which they at presentuse.

In the drawing, Fig. 1 illustrates diagrammatically, the circuits andapparatus at one terminal of an ordinary signal line on which the highfrequency system of the present invention has been composited, and Fig.2 illustratesdiagrammatically, the circuits at the opposite terminal ofthe same composited line but with certain modifications. Referring toFig. 1, the ordinary signal line to be composited is indicated at 1. Alow frequency line 2 of the usual type indicated as a telephone line isconnected by two-way repeating apparatus 3 including three-elementvacuum tube amplifiers of the thermionic type to incoming channel 4 andoutgoing 'channel 5 which are conjugately connected to compositedline 1. Both lines 1 and 2 are balanced with respect to their conjugateconnections by networks N each of which simulates the characteristicimpedance of the line which it serves to balrmits no carrier waves topass to.

ance for currents of all frequencies transmitted by that line. Sincehigh frequency currents are also to be transmitted over channels 4 and5, it is necessary to provide means to exclude them from line 2 and itsconnections which might otherwise dissipate their energy. Such highfrequency currents are excluded from the incoming side of the repeater 3by a low pass filter of the type described in U. S. patent to Campbell,No. 1,227,113 patented May 22, 1917. This particular filter consists ofa plurality of similar sections each having series inductance and shuntcapacity and its electrical characteristics are such that it transmitscurrents of the range of essential speech frequencies as, for example,100 cycles up to 2500 cycles with negligible attenuation. whilesubstantially excluding currents of all frequencies outside this range.High frequency currents in outgoing channel 5 are excluded from theoutgoing side of repeater 3 by reason of the well knownuni-directionally conductin characteristic of the thermionic ampli orconnected therein. The telephone station of line 2 may, therefore, carryon two-way communication over composited line 1 without interferencefrom the high frequency carrier currents.

Turning now to the high frequency circuits, 6 represents a thermionicoscillator which produces oscillations of base or control frequency, forexample, 5000 cycles. Oscillator 6 overloads a thermionic re ater '7connected to it by supplying to t e in- 7 will accordingly be ofnon-sinusoida wave form and are, therefore, the equivalent of a basefrequency current of sinusoidal wave form and a series of currents offrequencies which are multiples of the base frequency, that is, harmonicfrequencies. The repeater 7 will hereinafter be termed a harmonicproducer. The output circuit of hermonic producer 7 is provided with aplurality of tuned selecting circuits 8, 9, and 10 each comprising atuned path bridged across the output circuit and a similar] tuned loopinductively related thereto. gelecting circuits 8 select currents of thebase frequency and supply them throu h an amplifier and a filter 11 tooutgoing c annel 5 and thence to line 1. Filter 11 is of the Campbelltype and freely transmits oscillations of the base frequency, whilesubstantially extinguishing currents of frequency appreciably differingtherefrom. It therefore assists in preventing the transmission ofharmonic frequency currents from the harmonic roducer to channel 5 andit serves to exc ude the harmonic producer circuits.

lid

Selecting circuits 9 select currents of a particular harmonic as, forexample, 15,000 cycles and apply them as a carrier wave to circuit 12.In a similar manner, circuit 10 selects current of a different harmonicfre- %1]i1ency and transmits it to circuit 13..

ere may be as many of such selecting circuits each selecting a differentcarrier wave as is'desired, depending merely upon the number ofdifferent frequency carrier waves which are to be used.

A signal line 14 of the usual t pe, illustrated in the drawing as atelep one line has terminal connections similar to those oi line 2 withits repeater 3. However, the circuits 15 and 16 associated with line itare not repeater circuits in the sense that they serve merely totransfer speech currents to and from line 14. The function of circuit 15is to modulate carrier frequency oscillations in accordance with thecurrents transmitted from the telephone station of line It and totransmit such modulated carrier freuency oscillations to outgoingchannel 5. he function of circuit'16 is to select from incoming channel4, the particular modulated carrier waves which carry the returncommunication from the communicating station at the opposite terminal ofline 1 and to cause these selected modulated waves to reproduce in line14, speech current by which they were modulated. Circuit 15 includes abalanced modulator M of the type disclosed in United States Patent No.1,343,306 and a band filter 18 of the Campbell type, connected intandem. Speech frequenc currents are applied from line 14 to the ividedinput circuit of modulator M and carrier frequency oscillations areapplied by circuit 12 to the potentiometer 17 1n the common input pathof the modulator. As is well known, a modulator of this type will repeatno carrier current from circuit 12 in its input circuit, except at suchtimes as an electromotive force is .a plied across its divided inputcircuit w en it will re at the carrier currents in its output circuitswith a magnitude directly proportional to the amplitude of the waveapplied to the divided' input circuit. The Campbell wave filter 18transmits currents of a band fre-. quency which ma ran e either up ordown or both up and own rom the carrier frequency by the essentialspeech frequenc range which has been previously state Filter 18 servesto define, as desired, the range of the band transmitted to prevent lowfrequency currents from being repeated from line 14 throughthe modulatorM to channel 5 and to exclude from the out ut circuit of the, modulator,the foreign re quency currents of channel 5 which mi ht otherwise enterinto its modulating action and produce cross-talk at some receivingstation.

Circuit 16 includes a band filter 19 similar to 18, a balanced detectoror demodulator D, a thermionic amplifier and a low pass filter20 similarin electrical characteristic to the low pass filter of repeater 3.Filter 19 selects from incoming channel 4, the modulated oscillations ofa particular carrier frequency and applies them across the divided inputcircuit of balanced detector D. The common path of the output circuit ofdetector D includes a space current source and achoke coil in series.Shunted around the source and choke coil, is an alternating current pathcomprising 'a capacity element and a potentiometer. Across thepotentiometer, reinforcing oscillations of the carrier trequency areapplied by circuit 12, it being understood that the carrier frequncy ofthe modulated wave received by filter 19 is the same as that of themodulated wave trans mitted out to channel 5 by filter 18. The effect ofthe reinforcing oscillations is both to supply the unmodulated carriercomponent necessary for demodulation of the received modulated wavecomponents, and to augment the resulting low frequency signal currentwhich detector D supplies to the amplifier. Filter 20 preventstransmisson of high frequency currents to line 14.

There may, of course,-be any number of lines similar to line 14 and eachmay be connected to line 1 by circuits similar to those of'line 14: butdiderin principally in that they are each desi e to operate with anindividual carrier requency wave. It will also be clear that line 1serves to transmit ordinary speech currents from line 2, base frequencyoscillations from the harmonic producer 7 and modulated carrier wavesfor each of the lines 14.

Referring to Fig. 2, the opposite terminal of composited line 1 isillustrated as connected across a single distributing channel 21 insteadof to incoming and out oing channels, such as 4 and 5 of Fig. 1.owever,,eith,er arrangement may be used at both terminals of line 1.

Connected to channel .21 through a filter similar in characteristics tofilter 20, is a telephone signaling line 22 adapted to communicate overthe line 1 with line 2. The filter selects incoming speech frequencycurrents from channel 21 and transmits outgoing speech frequencycurrents to this channel.

A filter 23, similar to filter 11,-selects from channel 21, basefrequency oscillations and transmits them to an amplifier 24 b which aharmonic reproducer 25, similar in function and o cration to harmonicproducer 7, is overloa ed; The selecting circuits 26 which separate thevarious harmonic frequency carrier waves are series tuned loop circuitsand each corresponds in"- the frequency to which it is tuned to a selectin their design necessary for the different carrier frequenciesinvolved. Filter 27 is connected to a balancing network through abalanced transformer or hybrid coil with which are associated anincoming circuit 29 and an outgoing circuit 30. Incoming circuit 29includes a demodulator D having a constant current choke coil in itsoutput circuit and a filter 31 of characteristics similar to those ofelements D and 20 respectively in circuit 16 of Fig. 1. Reinforcingoscillations of carrier frequency are supplied to the demodulator by theproper selecting circuit 26 of the harmonic reproducer 25. It followsthat filter 31 transmits to line 28, the re roduced signaling currentsof line 14. The outgoing circuit 30 includes filters and a balancedmodulator M of the type de scribed in connection with circuit 15 ofFi 1. This modulator receives its carrier oscillations from the selectincircuit 26 which sup lies reinforcing oscillations to the demo ulator ordetector D of incoming circuit 29. These carrier oscillations areaccordingly modulated in accordance with the speech currents of line 28and are transmitted to line 1.

It will be noted that all of the carrier and reinforcing oscillations atthis terminal of line 1 are derived as harmonics of oscillations of baseuency transmitted over line 1 from the use-frequency oscillator and itsharmonic producer.

The same principles are applicable to radio signaling and to radioteledvnamic transmission generally in so far as the carrier frequencycommunications are concerned, and if the base frequenc and carrierfrequencies are sufficiently igh to be effectively radiated,transmitting and receiving antennae may be substituted in lieu of theline 1.

The term carrier wave. as used herein, includes, not only high frequencyoscillations of varying amplitude such as are commonly used in radiotelephony, but also high frequency waves or currents either merelyinterrupted or changed in frequency to denote signal or controlimpulses. This term is, however, not to be confused with high frequencyor radio frequency, since it is possible where ordinary telephony is notbeing transmitted, to transmit over a conducting line, a large number ofdifferent frequency alternating currents ranging from a few cycles persecond up toseveral thousand. each of these different frequency currentsserving as an individual carrier.

Although the features of this invention have been described as employedin particular circuits, it is to be understood that the invention is notto be limited thereto, but only by the scope of the appended claims.

\Vhat is claimed is:

1. The method of signalling between separated stations which comprisesroducing at all of the stations a wave of t e same frequency and ofsubstantially constant amplitude, deriving at each station from the waveproduced thereat a wave for the transmission of signals, producingamplitude variations in the wave so derived to represent the signals tobe transmitted, deriving at each fiation from the wave produced thereata wave of constant amplitude, and combining the last mentioned wave at astation with the signal modified wave of the same frequency incomingfrom another station to reproduce amplified signals.

2. The method of two-way carrier wave transmission between stationsassociated with a common carrier transmission circuit which comprisesreinforcing the waves received at each station by oscillations of thecarrier frequency of said waves and deriving all of said carrier wavesand said reinforcing oscillations-from oscillations produced by a singlesource.

3. A two-way carrier wave transmission system comprisin separatedstations, means at all of the stations for producing a wave of the samefrequency and of substantially constant amplitude, means at each stationfor deriving from the wave produced thereat a wave for the transmissionof signals, means for producing amplitude variations in the wave soderived to represent the signals to be transmitted, means for derivingat each station from the wave produced thereat a wave of constantamplitude, and means for combining the last mentioned wave at a stationwith the signal modified wave of the same frequency incoming fromanother station to reproduce amplified signals.

4. The method of two-way carrier signalling comprising modulating ineach of a plurality of stations a carrler wave by signals andsuppressing transmission of the unmodulated carrier component.transmitting from and receiving at each station the modulationcomponents of the signal-modulated carrier wave, separating the receivedwave from the wave being transmitted, detecting the receixed wave bycombining it with an unmodulated carrier wave of similar characteristicsto the wave used at a distant station for modulation, and deriving at agiven station the unmodulnted wave for both the modulating anddemodulating processes from the same carrier wave source.

5. The method according to claim 2 which comprises deriving a pluralityof different impressing reinforcing oscillations of the carrierfrequency of said received oscillations symmetrically upon said outputcircuit w ereby said reinforcing oscillations are prevented from beingtransmitted to said in at circuit or from said output circuit.

A multiplex transmission circuit having a plurality of receivingchannels each including an as mmetrically conducting demodulator, anmeans for supplying locally produced reinforcing oscillations to theout-' put circuit of said demodulator, whereby said reinforcingoscillations are prevented from being transmitted to said transmissioncircuit.

8. A system of two-way carrier transmission comprising a plurality ofstations, means at each of said stations for receiving carrieroscillations, means for reinforcing said received oscillations by localoscillations of the carrier frequency, and means for deriving all ofsaid carrier oscillations and said reinforcing oscillations from asingle oscillation source. v

9. A system according to claim 8 comprising means for composlting aplurality of two-way carrier transmissions on acommon transmissioncircuit.

10. In a two-way carrier telephone system a plurality of stations eachhavin a transmitting and a receiving circuit and a source ofoscillations, said sources of oscillations having the same frequency,means in'the transmittin circuit of each station formodulatingoscillations from the source at'said station in accordancewith speech, and means in the receiving circuit of each station forcombining speech-modulated carrier waves from another station withoscillations from the source at the station at which saidreceivingcircuit is located for producing interaction currents of speechfrequency.

11. A transmission line, a receiving channel and a transmitting channeleach connected thereto, a common source of oscillations of a singlefrequency for supplying carrier oscillations to said transmittingchannel and reinforcing oscillations to said receiving channel, andmeans for preventing interac tion of either of said channels upon saidsource or upon each other.

12. A receiving system comprising means for receiving high frequencyoscillations, a demodulator consisting of two three-element electricdischarge devices having separate input and output circuits andhavingtheir input circuits oppositely connected to said means, atransmission circuit, said discharge devices having output circuitsoppositely connected to said transmission circuit, and means forimpressing high frequency reinforcing oscillations on said outputcircuits.

13. In a carrier telephone system having stations, sources of carrieroscillations of the same frequency at respective stations, means at eachstation for modulating oscillations from the respective source by'voicecontrol for transmission, means at each station for combiningoscillations from the source at the respective station with the incominvoice modulated signal oscillation from t e other station, and meansresponsive to the combined oscillations for receivlng the detected voicemessa e.

In witness whereo I hereunto subscribe my name this 27th day ofSeptember A. D.,

EDWARD O. SCRIVEN.

